Irradiation for fast recovery of high power junction diodes

ABSTRACT

THE RECOVERY TIME OF A HIGH POWER JUNCTION DIODE IS SUBSTANTIALLY REDUCED AND TAILORED TO SEPECIFIC SPECIFICATIONS WHILE MAINTAINING WITHIN NOMINAL RANGES OTHER ELECTRICAL CHARACTERISTICS AND PARTICULARLY THE FORWARD VOLTAGE DROP OF THE DEVICE. THE DIODE IS IRRADIATED PREFERABLY WITH ELECTRON RADIATION WHICH PREFERABLY HAS AN INTENSITY BETWEEN ABOUT 1 AND 3 MEV. FURTHER PREFERRED, THE ELECTRON RADIATION IS APPLIED TO A DOSAGE LEVEL BETWEEN ABOUT 1 X 10**12 AND 1 X 10**14 ELECTRONS PER CM2.

May 7, 1974 s. TARNEJA ErAL 3,809,582

IRRADIATION FOR FAST RECOVERY OF HIGH POWER JUNCTION DIODES Filed March8, 1973 I WFiFiiiiiFiFiiPiYiiiP?)3223?;2 41W" United States PatentOflice 3,809,582 Patented May 7, 1974 3,809,582 IRRADIATION FOR FASTRECOVERY OF HIGH POWER JUNCTION DIODES Krishan S. Tarneja, Pittsburgh,and Frank V. Marcinko,

Uniontown, Pa., assignors to Westinghouse Electric Corporation,Pittsburgh, Pa.

Filed Mar. 8, 1973, Ser. No. 339,242 Int. Cl. H01l 7/00 US. Cl. 148-15 3Claims ABSTRACT OF THE DISCLOSURE The recovery time of a high powerjunction diode is substantially reduced and tailored to sepecificspecifications while maintaining Within nominal ranges other electricalcharacteristics and particularly the forward voltage drop of the device.The diode is irradiated preferably with electron radiation whichpreferably has an intensity between about 1 and 3 mev. Furtherpreferred, the electron radiation is applied to a dosage level betweenabout 1x10 and 1X10 electrons per cm FIELD OF THE INVENTION The presentinvention relates to the making of semiconductor devices andparticularly diodes.

BACKGROUND OF THE INVENTION A semiconductor diode is a two-electrodesemi-conductor device, having an anode and a cathode, which has markedunidirectional electrical characteristics. A junction diode is asemiconductor diode whose asymmetrical voltage-ampere characteristicsare manifested as a result of a PN junction formed at the transitionbetween N-type and P-type regions within the semiconductor wafer. Thisjunction may be either diffused, grown or alloyed.

A high power diode generally requires that one of the regions, usuallythe anode region, have a low impurity concentration, e.g., 1 10 to 1x10atoms per cm. This enables the device to withstand a high reverseblocking voltage without breakdown or punch-through by permitting a widespace charge region. The difiiculty with such devices has been the longreverse recovery time upon breakover into the conduction mode. That is,the time needed for the device to reestablish the blocking mode uponbreak-down or punch-through. Such recovery time is primarily dependentupon the recombination time of the minority carriers in the highlyresistive region, which as perviouslystated is usually the anode region.

In the past, the recovery time of both low and high power diodes hasbeen reduced by diffusion of gold into the highly resistive region and,in some cases, throughout the semiconductor body. However, gold isnotorious for its uncontrollability on diffusion. It is thereforedifficult to localize the gold diffusion with any precision within thesemiconductor body and/or to provide a uniform gold diffusion within thediffused regions of the body. Gold diffusion has therefore resulted inlow quantitative yields, particularly in high power junction diodes. Inaddition, gold diffusion has been found to increase the leakage currentat high temperatures through the PN junction of the diode.

It has been proposed to irradiate semiconductor devices for variousreasons. For example, it has been described in patent application Ser.No. 324,718 [W.E. 42,- 938], field Ian. 18, 1973, and assigned to thesame assignee as the present application, to bulk irradiate fastswitching thyristors to decrease the turnoff times. See also patentapplications Ser. No. 283,684, filed Aug. 25, 1972, Ser. No. 283,685,filed Aug. 25, 1972, Ser. No. 285,165, filed Aug. 31, 1972, Ser. No.343,070 [W.E. 43,860], filed Mar. 20, 1973, Ser. No. 354,620 [W.E.43,103], filed Apr. 25, 1973, and, Ser. No. 337,967 [W.E. 43,885], filedMar. 5, 1973, all of which are assigned to the same assignee as thepresent invention.

However, none of these previously described applicacations forirradiation in semiconductor manufacture involve diodes. To thecontrary, the mechanism postulated to occur on irradiation ofsemiconductor devices teaches that irradiation has utility insemiconductor manufacture only in gated devices to kill the gain of thedevice, or a portion thereof, to change the electrical characteristics.

SUMMARY OF THE INVENTION The present invention provides a junction diodesemiconductor body in which a recovery time is decreased whilemaintaining within nominal values other electrical characteristics,particularly forward voltage drop and high temperature leakage currents.The body is positioned with one major surface thereof and mostpreferably major surface adjoining the cathode region of the device forexposure to a radiation source, and thereafter the device is irradiatedby the radiation source.

Electron radiation is preferably used as a suitable radiation source inthe irradiation step because of availability and inexpensiveness.However, it is contemplated that any kind of radiation such as proton,neutron, alpha and gamma radiation may be appropriate, provided it iscapable of bombarding and disrupting the atomic lattice to create energylevels that substantially increase the recombination rate of theminority carriers without correspondingly increasing the carriergeneration rate.

Further, it is preferred that the radiation level of electron radiationbe between about 1 mev. and 3 mev. in intensity. Lower level intensityis generally believed to resalt in substantial elastic collision withthe atomic lattice and, therefore, does not provide enough damage to thelattice in commercially feasible times. Conversely, higher intensityradiation is believed to cause too severe lattice damage to thesemiconductor crystal to maintain certain other electricalcharacteristics of the device nominal values.

It has been found that an electron dosage between about 1 10 and 1X10electrons/cm. provides suitable radiation dosage. Lower dosage levelshave not been found to affect suitable reductions in recovery time.Conversely, radiation dosages above 1x10 electrons/cm. have notpermitted maintenance of other electrical characteristics andspecifically the forward voltage drop of the device within marketablevalues.

Other details, objects and advantages of the invention will becomeapparent as the following description of the present preferredembodiments and present preferred methods of practicing the sameproceeds.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, thepreferred embodiments of the invention and presently preferred methodsof practicing the invention are illustrated in which:

FIG. 1 is an elevational view in cross-section of a high power junctionsilicon diode being irradiated in accordance with the invention; and

FIG. 2 is a perspective of apparatus for performance of irradiation on aseries of junction diodes as shown in FIG. 1.

DESCRIPTION OF THE PREFERRED upwardly as shown in FIG. 1. To perform theirradiation, the electron dosage rate is measured by use of a Faradaycup in conjunction with an Elcon Charge Integrator and the radiationlevel adjusted to the desired dosage. Tray 30 with the bodies 10 inplace is then placed on the con- 5 EMBODIMENTS veyor belt 33 and movedby the conveyor 1n the direc- Referring to a junction Silicon dlodcWafer tion of the arrow through the electron radiation 20. The 01' bodyis Show a g opp major Surfaces 11 radiation dosage can also becontrolled by the speed of and 12 and curvilinear side surfac s 13. Dioy 1 the conveyor belt 33 as well as the intensity level of the hascathode region 14 and anode region 15 of impurities 1O eleetron di ti ofpp conductivity yp adjoining major surfaces By the irradiation as shownby FIGS. 1 and 2, the re- 11 and 12, respectively. Formed at thetransition between ver o r ti e of a high ower jun tion diode is regions14 and 15 in the interior of body 10 is PN unctypically reduced from 8--1 microseconds to 2 :05 tion 16. microseconds, while maintaining otherelectrical char- To provide electrical connections to the diode body, 15acteristics and particularly the forward voltage drop of metal contacts17 and 18 make ohmic contact to cathode th diod ithi k t bl l A n bappreciregion 14 and anode region 15 at major surfaces 11 and ated, awide range of reverse recovery times can be pro- 12, respectively. Toreduce channeling effects and atmosvided as d sired by us of theinvention. Such desired pheric effects on the diode operation, sidesurfaces 13 values for reverse recovery time cannot, however, be arebeveled y p etching and are Coated Wlth a Slut- 20 arbitrarily chosen.The reduction in reverse recovery time able passivating resin 19 such asa silicone, epoxy or varthat can be achieved by use of the presentinvention is nish composition. limited by the increases in forwardvoltage drop (V Irradiation is performed on diode body 10 by positionandreverse leakage current (1;) (hot) which can be ing major surface 11 forexposure to a suitable radiation tolerated. source. The diode body isthereafter irradiated by radia- To illustrate the operation of theinvention, the election 20 from the radiation source to a dosage levelsuftrical characteristics of ten (10) 1600 volt, silicon juncficient toreduce the reverse recovery time of the device tion diodes were measuredbefore and after irradiation to a desired value. to various dosagelevels. The diodes were N-type having As stated before, electronradiation is preferred for use a nominal diameter of 0.875 inch and anominal thickness as the radiation source because of availability andinexof 14 mils. The diifused cathode region has a diameter pensiveness.Moreover, electron radiation (or gamma of about 0.750 inch and a depthof 75 microns with a radiation) may be preferred in some applicationswhere surface impurity concentration of about 5 x 10 per cmfi. thedamage desired in the semiconductor lattice is to The diodes wereirradiated with 2 mev. electron radiation single atoms and small groupsof atoms. This is in conto various dosages from about 2X10 to 7X10electrast to neutron, proton and alpha radiation which causes trons/cmflarge disordered regions of as many as a few hundred A tabulation of themeasurements before and after atoms in the semiconductor crystal. Thelatter type of radiation are given in Table I.

TABLE I Before radiation After radiation Radiation VR at RT. VR at 150VF at 800 A. t" d dosage Va (R.'l.) VR (150 C.) VF 800 A. tn- Dlodenumber (volts) C. (volts) (volts) (e/emfl) (volts) (volts) (volts) (MS)1 1,600/ 1ma. 1,600/6 ma--.-- 1.16 9.5 195x10 1,600/ 1ma 1, 600/6 1, 02,8 2 1,600/ 1ma 1,600/6 ma 1.22 9.0 195x10 1,600/ 1ma- 1, 600/6 1.403.0 a 1,600/ 1ma. 1,600/7 ma. 1.17 9.0 7.s 10 1,600/ 1ma 400 13 1, ,4 41,600/ 1ma l,600/101na 1.20 8.5 7.8)(10 1,600/2 ma 600/10 1.9 1.5 a1,600/ 1ma 1,000 1 ma 1.13 9.0 150x10 1,600/ 11na.. 400 12 (e) 1.0 a1,600/ 1ma 1,600/6 'mn 1.1a 9.0 156x10 1,600/ 1ma 400 12 (0) 1.2 71,600/ 1'rna 1,600/6 "ma 1.17 9.0 2. 34x10 1,600/ 1ma 1, 600/10 (e)0.660 s 1,600/(1ma. 1,600/7 ma 1.14 9.0 2. 34 10 1,600/ 1ma 400 10 (2)0.900 9 1,600/ 1ma.--- 1,600/6 ma..-" 1.18 8.5 234x10 1,600/ 10 ma 20025 .3 ,660 10 1,600/ 1 ma.-- 1,600/6 ma..-.- 1.20 8.5 7.02X10 1,600/ 1ma1, 600/13 (e) 1430 a Reverse blocking voltage measured at roomtemperature. Reverse blocking voltage measured at 150 C. b Forwardvoltage drop measured at room temperature at 800 amps: d Reverserecovery time. c Too high to measure.

radiation may, however, be preferred for the radiation As can be seenfrom Table I, the reverse recovery time source in certain applicationsbecause of 1 ts better defined can be widely varied depending upon th lll i range and F l pz of lamce damaged crease in forward voltage drop (Vwhich can be tolert j i ti 15 i g P z g g gz' 232 3 1? ated in theparticular application. It can also be seen from .ecause o i s avalq 1 1y p q the table that the optimum results, keeping in mind the ages 1nshort periods of time. Su1table dosages of gamma 0 com romises with forwd h d V d radiation may require several weeks to be applied, while 1 I gV0 age 1 F) an reverse similar dosages of electron radiation can beapplied in ca age current L) were achleved by usmg minutes. ages ofabout 2x10 electrons/cm. and 8x10 elec- Referring to FIG. 2, apparatusis shown for performing trons/cm-zthe irradiation on the junction diodebody 10 as shown 5 To further illustrate the operation of the invention,irrain FIG. 1 with electron radiation. A conveyor belt 33 diations wereperformed with forty-five (45) 1200 volt/ is moved around roller orpulley means 32 which is ro- 1600 volt silicon junction diodes. Thediodes were N-type tated by a suitable power source (not shown)- 2mevhaving a nominal diameter of 0.914 inch and a nominal Van de GfaafiAccelerator 34 1S Posltloned to dlrect e163 thickness of 11 mils. Thediffused cathode region has a gaggi 20 Perpendlcular to conveyor belt 33to diameter of about 0.720 inch and a depth of 70 microns m A series ofjunction diode bodies 10 are positioned in with a surface i g 9 ZP 3 E51x10 planar array on a water cooled tray 30 having an electrof The f eswerefn'a late W1 e ectron statically attractive periphery 31. Bodies 10are positioned fadlatlon t0 Varlous Tadlatlon dosages of about X1Q 0with major surface 11 adjoining cathode region 14 facing 7X 10electrons/cm? The electrical characteristics were measured before andafter irradiation. The measurements are tabulated in Table H below.-

As the data from Table III shows, there was no appreciable change in theelectrical characteristics, and it was concluded that the inventionresulted in diodes that were TABLE II Before radiation After radiationRadiation VR(R.T.) Va at 150 tfl dosage VR(150 C.) V1800 a. tr Diodenumber (volts) 01 (volts) VF=(volts) (elem?) VR(R.'I.) (volts) (volts)(volts) s 1 1,600/15.0 ma 1,200/62.0 ma.-- 1.15 8. 6 8X10" 1,600/1.0 ma1,200/2.0 ma 1. 15 5. 5 2 1,600/4.0 ma 1,200/2.0 ma 1. 19 8. 6 8 1011,600/ 0.1 ma 1,200/2.0 ma.-. 1.17 5. 8 1,600/2.0 ma 1,200/3.0 ma 1.228.8 8X10 1,600/ 0.1 ma-.- 1,200/2.0 ma 1. 5.5 1,600/ 0.1 ma 1,200/3.0ma 1. 20 8. 4 8X10" 1,600/ 0.1 Ina.-. 1,200/2.0 ma-.- 1. 17 5. 41,600/4.0 ma 1,200/3.0 ma. l. 20 8. 6 8X10" 1,600/ 0.l ma--. 1,200/2.0ma 1. 17 5. 5 1,600/0.5 my.--.- 1,200/3.0 ma 1. 17 8. 8 8X10" 1,600/ 0.1ma--. 1,200/2.2 ma 1. 19 1,600/ 0.1 ma. 1,200/3.0 ma 1. 18 9. 0 8X10"1,600/ 0.1 ma 1,200 [2.2 ma... 1. 19 5. 5 1,600/l.5 ma 1,200/3.5 ma 1.17 9.0 8X10 1,600/ 0.1 ma 1,200/2.2 ma 1. 16 5.4 1,600/ 1 5 ma 1,200/5.0ma--- 1. 19 9.0 8X10" 1,600/0.5 ma 1,200/4.0 ma--. 1. 17 5.5 1,600/ 0.1ma- 1,200/2.0 ma. 1. 16 9. 4 8X10" 1,600/ 0.1 ma 1,200/2.0 ma- 1. 16 5.8 1 600/ 0.1 ma... 1 200/ ma--. 1. 15 9. 8 8X10" 1,600/ 0.1 ma..-1,200/2.2 ma 1. 15 5.5 1,540/15 0 ma 1,200/3 0 ma... 1. 19 8. 8 8X101,550/15.0 ma 1,200/2.0 ma 1. 16 5. 4 1,480/15 0 ma 1 200/2 5 ma.-. 1.17 8. 8 8X10" 1,480/15.0 ma..- 1,200/2.0 ma. 1.16 5. 4 1,600/15.0 ma1,200 .0 ma 1.17 9.0 8X10" 1,600/2.0 ma 1,200/20 ma 1. 16 5. 5 ,600/15.01118-..- 1,200/3.0 ma--- 1. 20 8. 8 8X10" 1,600/2.0 ma 1,200/2.0 ma 1.16 5.5 1,480/15.0 ma 1,200/3.0 ma 1. 20 9.2 8X10" 1,480/15.0 1118....1,200/2.0 ma 1.18 5.4 1,420/15.0 ma 1,200/3.0 ma- 1. 18 9. 6 8X10"1,420/15.0 ma- 1,200/2.0 ma- 1. 18 5. 8 1,400/15.0 ma 1,200/3.0 ma 1. 219. 0 8X10" 1,400/15.0 ma 1,200/2.0 ma 1.19 5.4 1,400/15.0 ma 1,200/3.5ma 1. 19 10.2 8X10" 1,400/15.0 ma 1,200/2.0 ma 1.16 6. 0 1,500/15.0 ma.1,200/3.0 ma 1. 20 8. 6 8X10" 1,500/15.0 ma. 1,200/2.5 ma. 1. 17 5. 01,360/15.0 ma. 1,200/l.5 ma 1. 19 10. 2 8X10" 1,360/15.0 ma....1,200/l.0 ma. 1. 16 6. 0 1,520/15.0 ma---- 1,200/2.0 ma 1. 20 9. 0 8X10"1,520/15.0 ma. 1,200/2.0 ma 1.18 5. 4 1,600/15.0 ma 1,200/1.5 ma-.. 1.209. 2 8X10" 1,600/2.0 ma 1200/20 ma 1. 18 5. 7 1,440/15.0 ma- 1,200/1.5ma 1. 19 9. 6 7X10 1,440/15.0 ma 1,200/5.0 ma 1.62 1.7 1,460/15.0 ma1,200/3.0 ma 1. 17 10. 0 7X10 1,460/15.0 ma. 1,200/5.5 ma 1. 60 1. 81,540/15.0 ma 1,200/2.0 ma 1. 17 9.4 7X10 1,540/15.0 ma 1,200/4.2 ma 1.62 1. 7 1,440/15.0 1,200/2.0 ma 1. 24 8. 2 7X10 1,420/15.0 ma 1,200/4.5ma 1. 75 1. 7 1 560/15 0 ma-. 1 200/2 0 ma 1. 23 8. 4 7X10 1,600/5.0 ma1,200/5.0 ma 1.76 1. 8 1,480/15 0 ma.--- 1,200/5 0 ma 1. 18 9. 6 7X101,480/15.0 ma 1,200/6.0 ma 1. 63 1. 8 1,500/15.0 ma 1,200/2.0 ma 1. 189. 4 7X10" 1,500/15.0 ma 1,200/4.5 ma. 1. 62 1. 7 1,500/15.0 ma1,200/l.5 ma 1.22 8.0 7X10 1,500 15.0 ma 1,200/4.5 ma 1.76 1.71,460/15.0 ma 1,200/3.0 ma 1. 22 8.6 7X10 1,460/15.0 ma 1,200/5.2 ma 1.75 1. 7 1,540/l5.0 ma 1,200/2.0 ma 1.21 8.6 7X10 1,540/15.0 ma 1,200/5.0ma 1.68 1. 7 1,560/15.0 1118--.. 1,200/5.0 ma. 1. 19 9. 0 7X10"1,560/l5.0 ma 1,200/6.0 ma. 1. 68 1. 7 1,560/15.0 ma 1,200/5.0 ma 1.209.4 7X10" 1,600/5.0 ma 1,200/7.5 ma 1.68 1.7 1,340/15.0 ma 1,200/3.0ma 1. 16 10. 0 7X10 1,340/15.0 ma 1,200/5.5 ma 1. 61 1. 7 1,400/l5.0 ma1,200/3.0 ma 1. 17 9. 8 7X10" 1,400/15.0 ma 1,200/5.0 ma- 1. 68 1. 81,460/15.0 ma 1,200/3.5 ma 1.17 8.4 7X10" 1,460/15.0 ma. 1,200/6.0 ma1.64 1.7 1,400/15.0 ma 1,200/3.0 ma 1.18 8. 6 7X10" 1,400/15.0 1118-.--1,200/5.0 ma..- 1. 64 1. 7 1,500/15.0 ma 1,200/3.0 ma 1. 18 8.6 7 101,500/15.0 ma 1,200/5.0 ma 1.67 1.7 1,400/15.0 ma 1,200/5.0 ma 1. 18 9.8 7X10 1,400/15.0 ma 1,200/7.0 ma 1. 65 1. 7 1,520/15.0 ma 1,200/3.0ma 1. 21 8. 8 7X10 1,520/15.0 ma 1,200/5.0 ma 1.65 1. 7 1,440/15.0 ma1,200/3.0 ma 1. 21 8.8 7X10" 1,440/15.0 ma 1,200/5.0 ma 1. 63 1. 81,520/15.0 ma 1,200/3.0 ma-.. 1. 21 8. 6 7X10 1,500/15.0 ma 1,200/5.0ma 1. 67 1. 8 1,600/15.0 ma 1,200/4.0 ma 1. 18 9.2 7X10" 1,600/5.0 ma1,200/5.0 ma 1.60 1.8

Reverse blocking voltage measured at room temperature.

b Reverse blocking voltage measured at; 150 C.

' Forward voltage drop measured at room temperature at 800 amps. 4Reverse recovery time.

TABLE III Annealing p at Diode time at Vn at R.T. Vn at 150 C. 300 8. in0. 250 0. (volts) (volts) (volts) olts) (us) After 64 hrs--- l,600/ 1 ma1,600/6 ma 1.28 2 8 do 1,600/ l ma 1,600/6 ma 1. 40 2 8 After 400 hrs1,600/ 1 ma 1,600/7 ma 1. 26 3 0 2 --do 1,600/ 1 ma. 1,600/5 ma 1 36 3 01 Reverse blocking voltage measured at room temperature. 2 Reverseblocking voltage measured at 150 C.

8 Forward voltage drop measured at 800 amps.

4 Reverse recovery time.

quite stable and maintained their characteristics even at hightemperatures (i.e., up to a maximum of 250 C.).

While presently preferred embodiments have been shown and described, itis distinctly understood that the invention may be otherwise variouslyperformed within the scope of the following claims. For example, theinvention has been particularly described with respect to siliconsemiconductor devices. It is contemplated that the present invention hasutility with other semiconductor materials such as germanium and galliumarsenide, although the particular radiation and intensity thereof andthe effectiveness of the invention will doubtless vary with thesemiconductor material.

What is claimed is:

1. A method of reducing the reverse recovery time of a junctioned diodecomprising the steps of:

(a) positioning a junction diode semiconductor body with a major surfacethereof to be exposed to a radiation source; and

(b) thereafter irradiating the diode semiconductor body with theradiation source to a dosage level between about 1 1O and 1 10electrons/cm. to reduce the reverse recovery time of the device. 2. Amethod of reducing the reverse recovery time of a junctioned diode asset forth in claim 1 wherein: the 5 radiation source of step (b) iselectron radiation.

3. A method of decreasing the recovery time of a junctioned diode as setforth in claim 2 wherein: the electron radiation has an intensitybetween about 1 mev. and 3 mev.

References Cited UNITED STATES PATENTS 9/1972 Bauerlein 250398 4/ 1972Coleman 250--398 JAMES W. LAWRENCE, Primary Examiner B. C. ANDERSON,Assistant Examiner US. Cl. X.R. 250-898, 400

